A. Field of the Invention
The device of the present invention relates generally to a new and improved high voltage electrical insulation system and to a new and improved method for making the insulation system.
B. Description of the Prior Art
Several different types of bulk high voltage insulation systems are used in the prior art, for example, as an electrical stress relief cone in a cable termination or as an insulating sleeve surrounding the electrically interconnected conductors of power cables in a cable joint. Such bulk insulation systems may broadly be categorized as being formed by either a unitary molded insulator or a layered insulator.
U.S. Pat. No. 2,748,184 (hereinafter '184 patent) discloses a well-known type of solid, preformed, unitary molded ceramic insulator 21, formed from wet process porcelain, and used as a stress relief cone. One advantage of unitary molded porcelain insulators is their short field installation time. However, porcelain insulators such as the porcelain stress relief cone 21 of the '184 patent exhibit relatively low dielectric strength, high dielectric loss and a high dielectric constant. In addition, such porcelain insulators are subject to thermal shock failure and to process defects, especially microvoids and cracks in relatively high defect populations. The defect population or number of microvoids and cracks present in the unitary molded insulators is generally proportional to the square of the cross-section of such unitary molded insulators. Thus, the larger unitary molded insulators are much more subject to damage and possible electrical destruction due to their proportionally larger defect population. Due to the high defect population of porcelain insulators, the electrical system in which a porcelain stress relief cone is used must be operated at a lower voltage level to prevent the possible destruction of the porcelain stress relief cone.
Furthermore, due to the mismatch between the relatively low coefficient of thermal expansion of a porcelain insulator as compared to the relatively high coefficient of thermal expansion of common plastic power cable insulation, the greater thermal expansion of the power cable insulation may result in either the cracking of the surrounding porcelain insulator or in the severe deformation of a power cable insulation. In order to prevent such damage, the porcelain insulator must be spaced sufficiently far from the cable insulation to prevent damage, which spacing is undersirable from an electrical stress standpoint. To eliminate this spacing, multiple thin layers of insulation from rolls of paper tape or from paper rolls may have to be applied to the power cable insulation, a task that normally requires a high degree of skill and a long installation time.
Another known type of unitary molded insulator is a cast epoxy resin insulator. The insulating element or sleeve 1 in the cable terminal illustrated in U.S. Pat. No. 3,049,581, the solid sleeve 2 illustrated the cable joint of U.S. Pat. No. 2,967,899 and the solid sleeve 12 illustrated in the cable joint of U.S Pat. No. 3,051,770 are examples of cast epoxy resin insulators. Such cast epoxy resin insulators, as discussed above with respect to the porcelain insulators, have relatively high defect populations by virtue of gas inclusion or heat cracking during formation and thus relatively low dielectric strengths.
A second category of bulk insulation systems are those formed by building up layers of insulating material. For example, vulcanizable insulating material in tape form may be formed by building up layer upon layer until an electrical insulator having a desired configuration is achieved. Subsequently, the layers may be subjected to heat to vulcanize the material. In addition, heat shrinkable thermoplastic material in tape form may be used to also form an electrical insulator by building up successive layers of tape until a desired configuration is achieved. Subsequently, heat is also applied to cause the thermoplastic material to, as far as possible, coalesce into a unitary structure.
Obviously, the disadvantages of these two types of bulk insulation systems are the relatively high probability of defects due to the inability to fully distribute the heat throughout the material. In addition, a relatively high degree of skill and a long field installation time is required to build up the desired insulator configuration by successive layers of tape.
Finally, other common types of layered electrical bulk insulation systems include paper tapes, paper rolls, cloth tapes and plastic films formed into a desired insulator configuration and secured by suitable mechanical fastening means. An example of a paper roll formed by cconcentrically wound layers of paper is illustrated in U.S. Pat. No. 3,322,884. An example of paper tape used to form an insulator is illustrated in the above-mentioned U.S. Pat. No. 3,051,770 in which the bulk insulation 19 is formed by building up successive layers of paper tape.
A major disadvantage of the layered type of electrical insulation system is the relatively high degree of skill and the relatively long installation time required to form an electrical insulator into a desired configuration. A significant advantage, however, of an electrical insulation system formed by oil impregnated or impregnable paper layers is the characteristic self-healing capability wherein localized hot spots or electrical damage within the oil impregnated paper insulation system due to high voltage electrical stresses thereacross may be healed or further damage retarded by the flow of a dielectric fluid, such as a dielectric oil, therethrough.